1. Field of the Invention
The invention relates to a method and apparatus for extracting materials from products and, more particularly, relates to a slotted scroll, single-screw counterflow extractor for removing soluble and dispersible materials such as sugar, color, and oil from products such as sugar beet peels, citrus peels, citrus concentrate, or the like.
2. Discussion of the Related Art
Screw operated devices are well known for removing soluble or dispersible materials from products. Such devices are typically called "presses" when used to remove mechanically dispersible materials from products, e.g., by compression, and are typically called "diffusers" when they are used to remove soluble materials from the products by dissolving the materials. The present invention is usable with both types of systems and will hereafter be designated an "extractor" for the sake of conciseness.
Extractors typically remove materials from products via a counterflow washing process in which an extraction medium such as water flows downwardly through an inclined housing while the product containing the materials to be extracted is conveyed upwardly through the flowing extraction medium. The counterflow extractor most commonly employed is the so-called "twin screw" extractor, an example of which is disclosed in U.S. Pat. No. 2,573,982 to Silver (the Silver patent). The extractor disclosed by the Silver patent includes a bifurcated trough-type housing having two parallel screw-type conveyors mounted therein in parallel with one another. Each conveyor has a rotatable shaft and a helical scroll which is formed from connected flights and which is mounted on the shaft. The scrolls of the conveyors present intermeshing flights which convey the products through the housing in unison without the product short circuiting between the flights.
In use, sugar beet cassettes or the like are fed into the lower end of the housing and are conveyed by the twin screws into and along a submerged course in an enriched solution designed to dissolve the sugar in the beets. The cassettes are conveyed progressively upwardly through the housing by the helical scrolls in counter-current relation to the liquid in the housing. Steam is delivered into and circulates through jackets surrounding the housing to maintain the desired temperature in the treatment zone. Fluid flow into and out of the housing is carefully regulated to maintain the liquid solution level in the housing to at least substantially fill the lower end of the housing and to fill more of the housing as may be required for sugar extraction.
The traditional twin screw extractor suffers from several drawbacks and disadvantages the most significant of which is that high levels of water are required for extraction because, in those portions of the extractor which are not submerged, the water and product flowing through the extractor simply flows between the two screws rather than through the products subject to diffusion. Significant diffusion takes place only in those portions of the screws which are at least partially submerged such that water must percolate through the fibrous products rather than simply run between the screws. This not only requires the use of high volumes of water, but also requires high energy expenditure to heat the water to the temperatures required for adequate diffusion.
The amounts of water or other liquids required for extraction can be reduced by using a single-screw extractor of the type disclosed in U.S. Pat. No. 4,363,264 to Lang et al. (the Lang et al. patent). The extractor disclosed in the Lang et al. patent includes an elongated trough-shaped housing in which is disposed a rotatable screw conveyor having a rotatable shaft on which is mounted a slitted helical scroll. The housing is inclined with a fibrous product inlet and liquid materials outlet located at its lower end and a fibrous product outlet and a wash water inlet at its upper end. The flights of the screw conveyor each have a plurality of concentric arcuate slits formed therein permitting the flow of water or other extraction medium through the flight and the product being treated.
In use, a product such as grapes to be subjected to the pressing or extraction process is fed into the lower end of the housing and conveyed upwardly through the housing by rotation of the screw conveyor. Heated water is fed into the upper end of the housing and flows through the product in a counter-current fashion, thus removing dispersible and soluble materials from the product. The treated product is then discharged from the upper end of the housing, and the extraction medium and extracted materials (which together form an effluent) are discharged from the lower end of the housing. Effluent flow through the housing is permitted by the circumferential slits in the flights.
The counterflow single-screw extractor disclosed by the Lang et al. patent exhibits fewer short circuiting problems than do twin screw extractors such as those disclosed by the Silver patent and thus require less extraction medium. In fact, the flow of the extraction medium through the flights can be reduced to the point that it is not necessary to submerge completely any portion of the screw. The extractor disclosed by the Lang et al. patent does, however, exhibit several drawbacks and disadvantages. Most notably, the fibrous product tends to compact while it is conveyed through the housing and to pile up on one side of the housing, thus permitting extraction medium to flow unimpeded along the other side of the housing. In addition, since the slits are circumferential and thus necessarily extend in the direction of screw rotations, these slits become clogged by the compacted product. The short circuiting problem is alleviated by periodically stopping the conveyor and permitting the product to fall by gravity back to the other side of the housing. However, the slit clogging problem can be solved only by periodically reversing the direction of the screw such that specially designed ribs bite into the compacted mass and redistribute the product in the housing, thus unclogging the slits. This necessarily requires the use of a bi-directional drive and also significantly decreases the rate at which extraction occurs.
The slit clogging problem encountered by the extractor disclosed in the Lang et al. patent is also encountered by other known single-screw counterflow extractors having perforated scrolls or scrolls with circular holes.
Another problem associated with the process disclosed in the Lang et al. patent is that it is usable with only a single slit configuration and thus cannot be tailored to meet the needs of different products. For instance, a product which is not ground to a uniform mass but which has some higher density, relatively large particles and some lower density, small particles is difficult to treat using the slit configuration disclosed by the Lang et al patent. When such a product is conveyed through the housing by the screw conveyor, the smaller, denser particles tend to migrate towards the bottom or outer portion of the trough, and the lighter particles tend to migrate to the center. Since all of the slits of the Lang et al. patent are of an equal diameter, the inner slits tend to become clogged by the larger particles while the smaller particles or fines wash through the outer slits.
Still another disadvantage of the system of Lang et al. patent is that it is incapable of performing a multistage extraction process using a single extractor. A multistage extraction process is one which treats a product in distinct stages using different extraction media and/or different extraction techniques at various stages. The extractor of the Lang et al. patent is incapable of performing such a process because it has only a single effluent outlet and is incapable of controlling the flow of extraction medium through the housing after it is fed into the extraction medium inlet.